The Insect in Winter: Snug as a Rugged Bug

By JANE E. BRODY

Published: January 6, 1998

THERE are good reasons that insects, not people, are the dominant form of multicellular life on earth. Not the least of them is the ability to survive extreme environmental stress, particularly the harsh winters in the temperate and polar regions.

While humans have only a limited number of options for keeping warm in winter -- donning extra clothing, staying in heated buildings and heading for hotter climes -- insects have evolved myriad strategies to survive winter's frigid conditions, including the ability to greatly lower their freezing points and, for some, to tolerate being frozen solid.

On Ellesmere Island, 500 miles from the North Pole, for example, an Arctic woolly bear caterpillar lies frozen for 10 months of the year, capable of surviving temperatures as low as 126 degrees Fahrenheit below the freezing point of water. With only two frost-free months each year to develop, this insect can take as many as 20 years to get through its life cycle, from egg to adult moth.

Another cryogenic wonder, the larva of a fly that forms a spherical gall, or tumor, on goldenrod, can survive with 70 percent of its body water frozen, even when temperatures dip to minus 90 degrees Fahrenheit, 122 degrees below freezing.

Why don't their delicate larval cells rupture when their body water turns to ice, and how do they avoid freezer burn? The answer, say entomologists who specialize in how insects make it through the winter, lies primarily in their ability to stoke their bodies with protective compounds, like the ones used to preserve sperm, embryos and blood cells when they are stored frozen in liquid nitrogen.

Unraveling the mysteries of how various insects cope with winter is giving researchers new handles on the biological control of insect pests, including ways to foil the cold-hardiness of crop-damaging pests while reducing agriculture's dependence on toxic chemicals. For example, Dr. Marcia Lee, a microbiologist at Miami University in Oxford, Ohio, said a number of laboratories were studying the bacteria and fungi that raise the freezing point of insect pests. The goal is to cause the insects to freeze to death during winter temperatures they would normally survive. The bacteria and fungi are the same ice-seeding microbes used to make snow on ski slopes.

Other researchers are cashing in on how the predators of crop pests get through the winter. Drs. Maurice and Catherine Tauber, husband-and-wife entomologists at the New York State College of Agriculture and Life Sciences at Cornell University, have shown that they could provide farmers with a ready supply of a valuable predator, the common green lacewing, a devourer of destructive aphids and caterpillars. By understanding the basic biology of lacewing dormancy, the Taubers devised a way to induce dormancy at will. They keep mass-cultured lacewings in cold storage in the off season, then artificially break the dormancy and have the insects produce as many eggs as they would have if they had not been stored.

Dr. Maurice Tauber said, ''Even with those insects that don't undergo a classic hibernation, we've learned how to induce cold hardiness and store them for months'' so they can be released over a field at the right time to attack an insect pest.

As cold-blooded creatures, arthropods that breed in the temperate and arctic zones have one all-consuming winter task: to find some way to keep from freezing to death. Dr. Marcia Lee's husband, Dr. Richard Lee, an insect physiologist at Miami University in Ohio, explained that most insects avoided freezing by behavioral adaptations. Some, like the monarch butterfly, migrate long distances to warmer territories, using the ever-shorter days of late summer as their clue to head south before the fall chill makes flying too difficult for an animal that gets sluggish in the cold.

Many others burrow deep into the soil or head for the bottom of a lake, below the frost line. Ants, for example, may dig down six feet, then form a tight huddle at the end of the tunnel to keep warm.

Crickets often hide out in a quiescent state in beetle burrows. Dr. Gilbert Waldbauer, author of ''Insects Through the Seasons'' (Harvard University Press, 1996) said the cecropia, a giant silk moth, ''just sits in a tough-walled silken cocoon with its metabolic rate about one-tenth that of normal, which allows it to survive 10 times longer'' while living off stored fat.

Many leaf-eating moths survive winter as eggs on the stems of plants that provide food for their larvae when the eggs hatch in spring. Thus, the eggs of gypsy moths can be found in felt-like clusters on tree trunks, and tent caterpillar eggs are found clustered around twigs, covered with a shiny brown varnish that helps protect them against the ravages of winter weather. Honey bees conserve body heat by clustering in a ball that keeps stirring all winter; bees on the outside keep warm by migrating toward the center of the ball.

And, as many a dismayed homeowner has discovered, cluster flies often come indoors in the fall, then spend winter as adults in nooks inaccessible to swatters. The American cockroach, known euphemistically as a ''water bug,'' stays warm by living in sewers in winter, while the smaller German cockroach thrives in heated kitchens.